The invention relates generally to lasers and laser systems, and more particularly to fiber lasers operating in the ultraviolet (UV) spectral range.
Ultraviolet laser sources having high power and high pulse energy for short nanosecond pulse widths are required for many important applications such as laser material processing and medicine, e.g., eye surgery. Presently, these applications primarily use excimer gas lasers because of the lack of other suitable laser sources. Excimer lasers can produce laser emissions at several main wavelengths, such as nanometer (nm) wavelengths of 351 nm, 308 nm, 248 nm, 193 nm and 157 nm. While being reliable laser sources, excimer lasers are bulky, require periodic service and, therefore, can have high ongoing costs of ownership. In addition high power excimer lasers often exhibit poor beam quality, use high voltage electronics and have low wall plug efficiency.
Fiber lasers are a relatively new type of laser source that are capable of delivering laser emission in a wide spectral range from infrared (IR) to UV. Fiber lasers can operate in continuous wave (CW) to ultra-short pulse modes with output power levels exceeding tens of kilowatts (kW). Commercially available fiber lasers operate in pulse and CW regimes, and at fundamental operating frequencies with micrometer (μm) wavelengths in the 1 μm, 1.5 μm and 2 μm ranges. Nonlinear frequency conversion can be used to generate higher order harmonics and shorter wavelengths. One approach to obtaining UV spectral wavelengths in the 190-196 nm range has been to use a fiber laser operating at a fundamental wavelength in the 1.5 μm range and non-linearly convert the fundamental frequency to its 8th harmonic. However, obtaining the desired average power level and pulse energy for nanosecond pulses in the higher harmonic spectral emissions has been a problem.
Several nonlinear optical effects, mainly Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Self Phase Modulation (SPM), as well as the bulk optical damage in a fiber core limit the pulse energy of a fiber laser. The fiber laser pulse energy is usually restricted to a fraction of a millijoule (mJ) for nanosecond (ns) pulse widths and the pulse peak power is restricted to tens of kW.
This peak power limitation may be overcome using ultra short pulse generation in the picosecond (ps) and femtosecond (fs) ranges due to higher optical damage threshold of the glass fiber in these ranges of laser pulse width. Ultra short pulse, high peak power fiber lasers can generate high average powers with close to transform limited pulses. However, even for transform limited operation, the ultra short pulse fiber lasers have relatively broad spectral line widths (this is a basic quantum mechanics uncertainty principle). For example, a 1 ps transform limited Gaussian shaped optical pulse has approximately a 3 nm spectral line width at a 1.5 μm wavelength. This broad spectral bandwidth limits efficient nonlinear frequency conversion to higher optical harmonics. In addition increasing the ultra-short laser pulse energy significantly increases the already high peak power which, in turn, triggers the detrimental nonlinear optical effects that impact the fiber laser power scaling.
Thus, power and pulse energy scaling of a fiber laser is a challenging task, particularly when applications require close to diffraction limited beam quality, a pulse width less than 10 ns, tens of watts of average power, and a polarized beam. Scaling a fiber laser pulse energy in the visible and UV spectral range through nonlinear frequency conversion is likewise limited by the low available pulse energy of the fundamental wavelength. With sub-mJ pulse energy at a fundamental wavelength of 1 μm, the pulse energy in the 5th or higher harmonics is of the order of a microjoule (μJ).
One approach to overcome these problems and scale the power and pulse energy to required levels in the UV range has been to use beam combining architectures (fiber bundling) that combines the outputs of several fiber laser sources to form a composite beam having increased average power and pulse energy with subsequent higher order nonlinear frequency conversion to the UV spectral range. For example, MOPA (master oscillator power amplifier) systems comprising erbium (Er) doped fibers synchronously seeded by the same master oscillator (MO) operating in the 1.5 μm range have been bundled to form a composite beam, and subsequently converted to the 193 nm spectral range. While fiber bundling can increase the pulse energy in the fundamental beam and high order harmonics, combining the outputs from bundled individual fiber lasers deteriorates the resulting laser beam and requires fine (nanosecond or sub-nanosecond) gating of the individual laser pulses to overlap. Additionally, the polarizations of the beams have to be aligned for efficient nonlinear harmonic generation which complicates spatial alignment of the individual fiber laser sources in the bundle. Another possibility is to combine multiple laser beams that have already been converted by the nonlinear frequency conversion to the UV range. However, this approach also requires several individual fiber laser sources, and is bulky, complicated and costly.
Other approaches to achieve UV fiber laser operation use optical frequency mixing of different pulsed MOPAs having different fundamental wavelengths, for example, one having a fundamental wavelength of 10YY nm and another having a fundamental wavelength of 15YY nm (or 21YY nm), to provide trains of optical pulses. The 10YY-nm pulses are frequency quintupled to a wavelength of 213 nm, and the 15YY nm (or 21YY nm) pulses are mixed with the 213 nm pulses to provide pulses having a wavelength of 193 nm. The 10YY nm and 21YY nm MOPAs include a fiber-laser and a bulk amplifier. However, this still requires two laser systems which need to be synchronized, properly triggered, and spatially overlapped in nonlinear crystals.
It is desirable to provide efficient power and energy scalable hybrid fiber laser/bulk crystalline solid-state amplifier systems capable of yielding a high pulse energy scalable to over 10 mJ, high average power scalable to over 100 W, output pulse widths controllable and variable from sub-ns to hundreds of ns pulse duration, and pulse repetition rates controllable from tens of Hz to over a MHz. It is to these ends that the present invention is directed.